The present invention relates generally to a stepper motor and more particularly to an acceleration shaping circuit for a stepper motor.
Stepper motors are operable in a low speed mode and a high speed, or slew, mode. To operate in the slew mode, the stepper motor must be accelerated from the low speed to the slew speed. This acceleration is generally known as "ramping".
With the presently available stepper motors and associated control electronics, ramping is either linear or exponential. But, due to variations in load, the speed/torque characteristics of the stepper motor, and the operative speed range of the stepper motor, there is no single preferred ramp shape. That is, the preferred acceleration is a function of system and load parameters and depends highly upon load friction and inertia.
In a principal aspect, the present invention is an improved acceleration shaping circuit for a stepper motor. The acceleration shaping circuit permits the operator of the stepper motor to vary or adjust the ramp shape and thereby substantially optimize the operation of the stepper motor in each particular application thereof.
The acceleration shaping circuit includes a power supply, a source circuit providing a substantially constant current when coupled to the power supply, a first and second voltage storage device, a first resistance element and first adjustable resistance element. The first voltage storage device is connected to the source circuit and charges in a linear fashion in response thereto. The voltage across the first voltage storage device therefore increases linearly.
The second voltage storage device is connectable to the power supply through the first resistance element. When connected, the second voltage storage device charges exponentially to provide an exponentially increasing ramp voltage.
The first adjustable resistance element has a pair of end terminals and a wiper. The end terminals are connected to the first and second voltage storage devices, respectively.
A variable ramp voltage is available at the wiper. The variable ramp voltage is adjustable between the substantially linear ramp voltage at one terminal end, derived from the first voltage storage device, and the substantially exponential ramp voltage at the other terminal end, derived from the second voltage storage device. By appropriately moving the wiper, i.e., by adjusting the first adjustable resistance element, the preferred acceleration voltage is provided and the acceleration ramp is optimally matched to the load.
It is thus an object of the present invention to provide an improved acceleration circuit for a stepper motor. Another object is a stepper motor acceleration circuit for producing a variable ramp voltage. It is also an object of the present invention to provide an acceleration shaping circuit wherein the output ramp voltage is adjustable between a linear ramp voltage and an exponential ramp voltage so as to accommodate varying applications of the stepper motor.
Still another object is an adjustable acceleration shaping circuit whereby the acceleration of the stepper motor to slew speed is substantially optimized. It is a further object to provide a stepper motor acceleration shaping circuit whereby the transistion or acceleration time is substantially minimized. Yet another object is a simple, readily manufactured and maintainable acceleration shaping circuit.
These and other objects, features, and advantages of the present invention are described or apparent in the following detailed description.